1. Field of the Invention
The present invention relates to a process for preparing 5-ethylidene-2-norbornene (hereinafter referred to as "ENB") with high quality. Particularly, the present invention relates to a process for preparing ENB by isomerizing 5-vinyl-2-norbornene (hereinafter referred to as "VNB") containing a certain amount of 4-vinylcyclohexene (hereinafter referred to as "VCH") in the presence of a specific solid base catalyst.
2. Background of the Invention
ENB is the most promising compound as a third monomer of a terpolymer of ethylene, propylene and dienemonomer (EDPM rubber) and prepared by isomerizing VNB in the presence of a catalyst. VNB is produced by reacting 1,3-butadiene and cyclopentadiene.
As the isomerization catalysts, there are known liquid bases such as mixtures of an alkali metal hydroxide and an aprotic organic solvent, of an alkali metal amide and an amine and of an organic alkali metal compound and an aliphatic amine. Such liquid bases, however, do not have enough catalytic activity so that a large amount of the expensive catalyst should be used. Further, since separation and recovering of the catalyst component from an reaction mixture are very difficult, the process requires complicated separation and recovering steps and consumes a large amount of energy.
There are also known solid isomerization catalysts, for example, an alkali metal carried on an anhydrous carrier with a large surface area (e.g., activated carbon, silica gel, alumina and the like) (cf. J. Am. Chem. Soc., 82, 387 (1960)). The solid catalyst, however, has unsatisfactory handleability and less safety because it is ignited and loses activity upon contact with air. This is because the alkali metal is only finely dispersed on the carrier. Further, the solid catalyst has insufficient isomerization performance.
The present inventors have proposed a solid base catalyst for isomerizing olefins such as VNB, which does not suffer from the drawbacks of the conventional isomerization catalysts. The proposed catalyst is prepared from alumina, an alkali metal hydroxide and an alkali metal, or from water-containing alumina and an alkali metal. The solid base catalyst has higher stability to air and excellent isomerization activity of olefins such as VNB than the alkali metal dispersion catalyst (cf. Japanese Patent Publication Nos. 35264/1974 and 29058/1980).
As a result of further study on the isomerization of VNB with the solid base catalyst, it was found that VNB with decreased quality was produced in some cases. When such VNB with decreased quality is used as a third monomer of EPDM, the of a polymerization catalyst is deteriorated and a molecular weight distribution is undesirably varied.